1. Field of the Invention
The invention is directed to an improved ceramic circuit card construction and a method for producing such a card. The improved ceramic circuit card provides increased adhesion strength for circuit pads fabricated thereon, and in particular copper input/output (I/O) circuit pads which are fabricated on dielectric layers of the circuit card and make use of multiple vias filled with conducting material to increase adhesion.
2. Description of Related Art
Ceramic circuit cards are complex, multi-layered circuits and include multiple I/O circuit pads that are fabricated onto an alumina ceramic base layer using thick-film copper inks. Such circuit cards are often employed as sensing, guiding or radar circuits in projectiles such as missiles where the cards are subjected to high and low temperature extremes as well as considerable shock and vibration. The ceramic circuit cards must have sufficient adhesion strength to withstand these conditions.
On conventional thick-film ceramic circuit cards, copper I/O pads are fabricated directly on relatively large area alumina ceramic. With this construction, the adhesion strength of the I/O pads on the ceramic is dependent upon the nature of the mechanical and chemical bond at the copper ink-to-ceramic interface. During the fabrication of the multiple circuit layers, these cards are repeatedly exposed to furnace temperatures of approximately 900.degree. Celsius, often causing the copper ink-to-ceramic adhesion to degrade to undesirable levels. Exposure of fabricated cards to plating chemistry whereby the cards are further processed by plating them with nickel and gold to make them more solderable may also act to reduce I/O pad adhesion.
Due to the loss of adhesion strength during the fabrication process, the finished ceramic cards do not consistently exhibit sufficiently high average tensile pull adhesion characteristics for use of the ceramic cards in extremely high and low temperatures and in high vibration environments. Moreover, with the previous design and manufacturing process, there is a distinct possibility that variations in the manufacturing process will result in some lots of cards being produced having I/O pad adhesion distributions below the minimum level, thus increasing the risk that these I/O pads will peel off of the ceramic during assembly, testing or in flight.
It is, therefore, an object of this invention to overcome one or more of these problems.